With the method disclosed in West Germany DE-AS No. 22 54 146, the surface of the porous body remains completely impregnated with liquid, thereby giving the exterior of the body a dark and moist looking appearance even after the impregnating liquid has hardened. This external alteration of the body is undesirable, especially in the preservation of stones.
West German DE-OS No. 19 38 077 discloses the impregnation of clay stones with resin. This method has been used for the treatment of stone monuments to prevent exterior pollutants penetrating into the stone. However, where stone pollutants and water are present in the stone interior prior to treatment, the treatment prevents water entrained pollutants getting out of stone and as a result the pollutants are deposited to to a greater degree behind the protective layer and their destructive action increases at this point. Also during a frost, ice crystals may form behind the protective layer, which after awhile, causes the outer sealed layer to chip off.
To prevent the action referred to in the above paragraph, a prior art practice has been to impregnate the outer layer of stones with a medium which does not form a complete water and vapor barrier, but only has a water-repelling action, so that release of vapor from the stone continues to be possible. But as long as vapors can pass through the impregnated layer from the inside out, invasion of harmful gases is also permitted, so that protection of the stone is conditional.
Also it has been proposed to impregnate a stone with hardenable resin completely. To this end the stone was transported from the site to a pressure chamber, so that this method is not feasible with stones of very large dimensions. For the treatment of large stones on the spot it has been proposed in West Germany DE AS No. 11 73 380 to envelop the stone with a plastic foil and to suck liquid resin into the stone thus sealed by applying a vacuum. Apart from the fact that such sealing requires a high labor cost and in the case of complicated bodies is often not complete, a plastic layer will form between the foil and the stone surface, in particular in depressions, which considerably alters the outward appearance of the stone.